Mechanical repair for a honeycomb panel

ABSTRACT

A method of reinforcing a sandwich panel having opposed first and second skins spaced apart by a core having a thickness &#34;F&#34; in a manner to preserve the spacing between the skins, including the steps of boring an in-line hole through the panel to provide a hole in both skins and the core of a diameter &#34;A&#34;, enlarging the hole in the first skin to a diameter &#34;B&#34;, inserting a tubular sleeve into the in-line hole, the sleeve having a length substantially equal to &#34;F&#34; and having an external diameter greater than &#34;A&#34; and slightly less than &#34;B&#34;, the first end of the sleeve contacting the inner surface of the second skin, and flaring the second end of the sleeve to an external diameter greater than &#34;B&#34; so that it engages the inner surface of the first skin, and inserting a bolt through the sleeve, the bolt having a head to contact the external surface of the first skin and applying a nut to the bolt to contact the external surface of the second skin so that the skins are held in fixed position by the bolt.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is not related to any pending patent applications.

CROSS-REFERENCE TO MICROFICHE APPENDIX

This application is not referenced in any microfiche appendix.

BACKGROUND OF THE INVENTION

Honeycomb panels are frequently used in the aircraft industry and inother industrial applications. By "honeycomb panel" is meant a panelformed of a relatively thin inner and outer skin secured to a core.While a honeycomb panel is the usual and typical application of thepresent invention, the invention is not limited to a panel in which thecore is of honeycomb configuration but is applicable to any panel havinga first and a second skin bonded to a core which may be more generallycalled a "sandwich panel". The purpose of the invention is to providemeans for structurally reinforcing any such panel, that is, any panelhaving a first and second skin separated by a core.

A specific example of the application of this invention is forreinforcing honeycomb panels utilized on aircraft, such as in structuresattached to an aircraft jet engine to reduce engine noise. Because ofthe high forces, drastic temperature differentials and vibration, theskin of honeycomb panels can become separated from the core or the coreitself may destruct. To repair a honeycomb panel in this environment thepresent procedure is to use rods that are welded to join to the twoskins together to thereby entrap the core. This procedure workssuccessfully but is time consuming, particularly due to the high degreeof cleanliness which must be established before effective welding can beaccomplished. The tail pipe environment in which these panels are usedexposes them to oil, soot, and other materials. Cleaning the panels toput them in condition to receive effective and permanent welds isdifficult. The invention herein provides a method of reinforcinghoneycomb panels in which it is not necessary to thoroughly clean thepanels as is required with welding procedures.

The invention herein provides a mechanical system for reinforcing skinsbonded to opposed sides of a core material, such as a honeycomb corematerial. Specifically, the invention herein provides a method andapparatus for installing a Bolt in a sandwich panel in a way to resistdelamination of the skins from the core.

For background information relating to structures utilized to reinforcesandwich panels having first and second skins with a core therebetween,reference should be had to the following United States Patents.

    ______________________________________                                        PATENT NO.                                                                             INVENTOR   TITLE                                                     ______________________________________                                        3076668  Famely     Fittings For Retaining Cylindrical                                            Members In Place                                          3468091  Gerhard    Wall Fasteners                                            3526072  Campbell   Load Distributing System For Panels                                           Incorporating Honeycomb Core                              4048898  Salter     Fastener For Multi Metal Stack-Ups                        4089247  Dahl et al Blind Fastener                                            4499647  Sakamura et al                                                                           Blind Nut Fastening Method                                4656806  Leibhard et al                                                                           Expansion Anchor Assembly                                 4717612  Shakelford Fasteners For Honeycomb Structures                        4934861  Weeks et al                                                                              Attachment Apparatus For External                                             Stores On Thin-Wall Poles                                 5040917  Camuffo    Device For Fixing Mechanical Parts                                            To The Body Of A Motor Vehicle                            5205689  Fischer    Expansible Plug                                           ______________________________________                                    

BRIEF SUMMARY OF THE INVENTION

The invention disclosed herein relates to a method of reinforcing asandwich panel having opposed first and second skins spaced apart by acore. While the invention is applicable to any type of panel havingopposed skins and a core, it is particularly applicable for use with ahoneycomb panel assembly. Honeycomb panels are frequently employed inthe aviation industry since they provide an exceedingly high strength toweight ratio. The skins of honeycomb panels can be formed of any stiffmaterial and are typically of plastic, fiberglass or metal. Metalhoneycomb panels are frequently employed in the aviation industry inharsh environments, such as to provide sound reduction structures thatare mounted at the aft end of jet aircraft engines. The sandwich panelsin these environments are subjected to dramatic temperaturedifferentials, pressure and vibration. Consequently the panels candelaminate, that is, one or both of the skins detaches from the panelcore or the core itself could degrade. Repairing a delamination ordamage of this type in this environment is difficult. The inventiondescribed herein provides an improved method of accomplishing repairs orreinforcement of a sandwich panel, such as a honeycomb panel.

The first step of the method requires boring an in-line hole through thepanel to penetrate the first skin, the core, and the second skin. Next,the hole in the first skin is enlarged to a slightly increased diameter.Third, a tubular sleeve that has circumferential integrity throughoutits length is then inserted in the in-line hole, the sleeve having afirst end and a second end and a length substantially equal to thethickness of the panel core. The sleeve has an external diameter greaterthan the in-line hole in the second skin but slightly less than theenlarged hole in the first skin. The first end of the sleeve therebyengages the inner surface of the second skin. In the fourth step, thesecond end of the tubular sleeve, after being inserted within thedrilled hole in the panel, is flared by cold flowing, that is, withoutdestroying the circumferential integrity of the sleeve so that theflared diameter of the second end of the sleeve is enlarged to a sizegreater than the diameter of the enlarged hole formed in the first end.In this manner, a sleeve is provided within the confines of the core ofthe skin that is tubular, that is, has an opening through it, with afirst end that engages the interior surface of the second skin and asecond end that engages the interior surface of the first skin. Thesleeve thereby spaces the first and second skins apart at the samespacing as provided by the core. Thereafter, a bolt may be positioned inthe sleeve, the bolt having a head at one end with or without a washerthat engages the external surface of the first skin and a nut with orwithout a washer, the washer being integral or separate at the other endthat engages the external surface of the second skin so that, when thenut is tightened on the bolt, the first and second skins are held infixed relationship to each other with the spacings between the interiorsurfaces defined by the length of the sleeve to thereby maintain thethickness of the skin at the area of repair in conformity with thenormal skin thickness. Instead of employing a bolt, a rivet, or anotheritem may be utilized to secure the first and second skins to each other,the rivet extending through the tubular sleeve with an enlarged diameterportion on each end to engage the exterior surfaces of the first andsecond skins.

A repair system to accomplish this method includes the use of a sleevein combination with a bolt having a frusto-conical portion adjacent oneend. The bolt, when inserted in the sleeve after it is positioned withinthe drilled hole in a sandwich panel, functions to expand one end of thesleeve. The frusto-conical portion of the bolt may either be integralwith the bolt or may be a separate element slidably received on thebolt.

A better understanding of the invention will be obtained from thefollowing detailed description of the preferred embodiment, taken inconjunction with the attached drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view showing a sandwich panel, such as a honeycombpanel, in cross-section and showing a tubular sleeve, a bolt and a nutas used in the system to practice the method of the invention.

FIG. 2 is a cross-sectional view of the panel of FIG. 1 showing thesleeve in position within the panel and showing the bolt in position asemployed to reinforce the panel. The bolt of FIGS. 1 and 2 has anintegral flare portion adjacent the bolt head.

FIG. 3 is an exploded view, similar to FIG. 1, showing a sandwich panelin cross-section, the panel having an in-line hole drilled therethroughwith the hole in the panel first skin being slightly enlarged andshowing a tubular sleeve dimensioned to be received in the in-line holewithin the panel. A flaring washer that is slideably receivable on abolt is shown.

FIG. 4 shows the panel in cross-section with the sleeve positionedwithin the interior of the panel. The flaring washer is illustrated asit is positioned to be inserted into the panel by means of a bolt toflare one end of the sleeve.

FIG. 5 is a cross-sectional view of a sandwich panel as shown in FIG. 3showing the sleeve, bolt, flaring washer and nut in position toreinforce the panel.

FIG. 6 is an elevational cross-sectional view of a sandwich panel asshown in FIG. 5 but with the flaring washer removed. FIG. 6 shows howthe method and the system of the invention may be employed wherein theflaring washer is utilized in the installation of the sleeve but isremoved prior to the final tightening of the bolt that serves toreinforce the panel.

FIG. 7 is an elevational cross-sectional view of a sandwich panel havingthree skins, that is, a first and second skin which may also be termedan inner and outer skin, and an intermediate skin. In the embodiment ofFIG. 7 a single bolt is employed to retain the inner and outer skins inposition with respect to each other and with respect to the center skin.In this figure the spacing between the first skin and the intermediateskin is different than the spacing between the intermediate skin and thesecond skin.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to FIG. 1, the numeral 10 indicates a sandwich panelwhich is typically in the form of a honeycomb panel. Panel 10 is formedof three basic components, that is, core 12 which is usually in the formof honeycomb material, first skin 14 and second skin 16. Panel skins 14and 16 are typically relatively thin sheets of stiff material such asplastic, fiberglass, wood or metal. The present invention isparticularly useful in the aircraft industry and in these applications,skins 14 and 16 are typically formed of metal, the metal usually beingaluminum, aluminum alloy or, for particularly harsh environments,titanium, titanium alloy or steels. Skins 14 and 16 are bonded to core12. In some applications, particularly where the panel 10 is subjectedto stress, vibration, great temperature variations and so forth, one orthe other of skins 14 and 16 can sometimes delaminate from core 12, thatis, the bonding between the core and the skin fails causing one or bothof the skins 14 and 16 to separate from core 12 or the core itself candegrade.

For reference to a sandwich panel 10, and particularly a honeycomb panelthat is exemplary of the type of panel in which the invention ispracticed, reference may be had to U.S. Pat. Nos. 5,064,493 entitled"Method of Producing Curved Honeycomb Core Material Having Crimps In OneEdge" and 5,126,183 entitled "Curved Paneling Including Honeycomb CoreMaterial Having Crimps In One Edge", which are incorporated herein byreference. These patents teach a method of construction of honeycombpanels and particularly, the method of constructing honeycomb panels ofa curved configuration.

Assuming it is the objective of a practitioner to reinforce panel 10such as to remedy delamination or to prevent the possibility ofdelamination or core degradation, the method and the system of thisinvention can be employed.

First skin 14 has an outer surface 14A and an inner surface 14B. In likemanner, second skin 16 has an outer surface 16A and an inner surface16B.

The first step in practicing the method of this invention is to drill anin-line hole through the panel, that is, through first skin 14, core 12and second skin 16. The in-line hole is indicated by the dotted lines18, the in-line hole having a diameter "A". The in-line hole 18 forms ahole 20 of diameter "A" in second skin 16 and forms an in-line hole inthe first skin 14 of the same diameter. The second step in the method isto enlarge the hole in skin 14 and core 12 to a larger diameter hole 22.The diameter of enlarged diameter hole 22 in skin 14 is indicated by theletter "B". The drilling operation to provide hole 22 in skin 14 iscarried out to the full depth of core 12 if core 12 is solid. If core 12is honeycomb material the core space where the in-line hole is drilledmay be clear in which case the second drilling operation may need onlypenetrate first skin 14.

The thickness of core 12 between first skin inner surface 14B and secondskin inner surface 16B is indicated by the letter "F".

The third step in the method of practicing the invention is to insert atubular sleeve 24 into hole 22 formed in first skin 14 and into the hole26 of the same diameter formed in core 12, the sleeve havingcircumferential integrity throughout its length. The external diameterof tubular sleeve 24 is slightly less than "B" so that the sleeve willslide freely through opening 22 in first skin 14 into opening 26 in core12. The length "D" of sleeve 24 is substantially the same as dimension"F", that is, the spacing between the interior surfaces of first andsecond skins 14B and 16B. The internal diameter of opening 28 of sleeve24 is preferably substantially equal to "A", that is, the diameter ofthe first drilled in-line hole and the diameter of hole 20 in secondskin 16.

Sleeve 24 is positioned within panel 10 as shown in FIG. 4. Tubularsleeve 24 has inner end 30 that abuts second skin inner surface 16B andhas an outer end 32 that, as shown in FIG. 4, is in the plane of thefirst skin inner surface 14B.

The second element employed in practicing the method of this inventionis bolt 34 having a head 36 at one end and a threaded area 38 adjacentthe opposite end 40. Bolt 34 of FIGS. 1 and 2 has, adjacent head 36 anintegral flange portion 42. Integral flange portion 42 increases indiameter towards head 36 to a diameter "E" that is substantially equalto "B". Bolt 34 also includes an integral short length frusto-conicalportion 44 that preferably is of a length substantially equal to thethickness of first skin 14.

With sleeve 24 positioned within core 12 of the sandwich panel, bolt 34is extended through the sleeve and nut 46 is threaded onto the bolt asshown in FIG. 2. When nut 46 is tightened the bolt integral flangeportion 42 is forced into tubular sleeve outer end 32 to swedge, thatis, enlarged by cold flowing the sleeve and portion without destroyingthe circumferential integrity of the sleeve so that the inner end 32increases in diameter as shown in FIG. 2. As integral flange portion 42is fully forced into sleeve 24, inner end 32 increases in diameter sothat the internal diameter at the sleeve inner end 32 is substantiallyequal to "E" with the external diameter of the inner end 32 after theswedging operation, being greater than "B". In this way, the enlargeddiameter inner end 32 of sleeve 24 engages first skin internal surface14B in the same way that the inner end 30 of the sleeve engages secondskin internal surface 16B. In this way sleeve 24 forms a structuralspacer holding the skins apart the distance "F", that is, the normalthickness of core 12.

It can be seen with reference to FIG. 2 that with nut 46 tightened firstskin 14 and second skin 16 are held in fixed relationship relative tocore 12 and held in fixed spacing with respect to each other with thecore therebetween. Thus, the method of this invention employing theenforcing system serves to capture the opposed skins of a sandwichpanel, and particularly a honeycomb panel, to support them in theirpre-established spacing and in a manner that the skins are not free toseparate from each other or from the core.

After bolt 34 has been installed and the inner end 32 of sleeve 24 isexpanded as illustrated, as shown in FIG. 2, the bolt may be removed anda different bolt employed or the opening provided by sleeve 24 afterbolt 34 is removed can be used such as to attach the panel 10 to anotherobject. However, if bolt 34 is removed it must be replaced by some otherelement to retain the first and second skins relative to each other withthe core trapped therebetween and for successfully reinforcing thepanel.

The drawings do not show a washer received on the bolt before nut 46 isapplied but such could be a standard procedure.

FIGS. 3, 4 and 5 show an alternate embodiment that is different in onlyone basic respect, that is, in the alternate embodiment bolt 48 is notprovided with an integral flange, such as integral flange 42 of FIGS. 1and 2, but instead, an additional element is employed. The additionalelement is a flare washer 50. Flare washer 50 has an integral washerportion 52 (See FIG. 4) and inwardly and integrally of it, a flareportion 54, the flare portion having external dimensions substantiallythe same as the integral flare portion 42 of bolt 34 previouslydescribed. Flare washer 50 has an opening 56 therethrough by which it isslideably received on bolt 48.

To practice the method of the invention using the embodiment of FIGS. 3,4 and 5, the same procedure is employed except that flare washer 50 ispositioned on bolt 48 before the bolt is inserted into the sleeve andnut 46 tightened to the position as shown in FIG. 5. Flare portion 54performs the same function as previously described, that is, it flaresby cold flowing the inner end 32 of tubular sleeve 24 without destroyingthe sleeve circumferential integrity so that the inner end is of largerdiameter than opening 22 in first skin 14. Head 58 on bolt 48 isdifferent than that of FIGS. 1 and 2, head 58 being of reduced and morestreamlined in size. Flare washer 50 may remain as a permanent part ofthe structural reinforcing of panel 10 as shown in FIG. 5 or, afterflaring sleeve 24, nut 46 can be removed, allowing bolt 48 and flarewasher 50 to be removed, after which bolt 48 may be reinstalled as shownin FIG. 6. This permits flare washer 50 to be reused in practicing themethod of the invention.

FIG. 7 shows the embodiment of the invention wherein the sandwich panel10A is formed of three skins, that is, a first skin 14 and a second skin16 as has been previously described, and an intermediate skin 60. Afirst core 12 is positioned between first skin 14 and intermediate skin60 and second core 12A is positioned between intermediate skin 60 andsecond skin 16.

To repair a three skin sandwich panel a first hole is drilled throughthe entire panel, the first drill hole penetrating skins 14, 60 and 16and cores 12 and 12A. The first hole has a diameter "A" which is thesame diameter as appears in the hole 62 formed in intermediate skin 60.Subsequently holes 60 and 62 of enlarged diameter "B" are drilled infirst skin 14 and second skin 16 respectively, but not in intermediateskin 60.

A first sleeve 24A is inserted through hole 64, the length of the sleevebeing substantially equal to the thickness F₂ of core 12. A secondsleeve 24B is inserted through hole 66, the length of second sleeve 24Bbeing substantially equal to thickness F₁ of core 12A, sleeves 24A and24B each have circumferential integrity throughout their lengths. Afirst flare washer 50A is positioned in alignment with the drilled holesand in contact with the exterior surface of first skin 14. In likemanner, a second flare washer 50B is positioned in alignment with thedrilled holes and in contact with the exterior surface of second skin16. Bolt 68 is inserted through aligned flare washer 50A, sleeve 24A,hole 62 in intermediate skin 60, sleeve 24B, and flare washer 50B.Thereafter nut 70 is attached to the bolt and tightened. The bolt head72 engages flare washer 50A, forcing it into sleeve 24A to outwardlyexpand by cold flowing the end thereof so that it engages the interiorsurface of first skin 14. That is, the circumference of tubular sleeve24A adjacent first skin 14 is expanded to an external diameter greaterthan "B" to thereby engage the interior surface of first skin 14. Inlike manner, flare washer 50B expands by cold flowing the end of tubularsleeve 24B to a diameter greater than "B" to thereby engage the interiorsurface of second skin 16. As nut 70 is tightened, sleeves 24A and 24Bbear against the opposed surfaces of intermediate skin 60 so that thethree skins are held in fixed relationship with respect to each other,maintaining the thickness "F₁ " of core 12A and "F₂ " of core 12.

In the embodiment of FIG. 7, the flare portion of each of washers 50Aand 50B is precisely frusto-conical, not having a streamlined flaringportion as illustrated in FIG. 4. The function of flare washers as shownin FIG. 7 is the same as that previously described with reference toFIGS. 3 and 4, that is, the flare washers serve to expand an end of asleeve so that the expanded end fits against the interior surface of theadjacent skin.

The method and system of this invention provide an effective andinexpensive way of reinforcing a sandwich panel, and particularly ahoneycomb-type of sandwich panel utilizing a relatively inexpensivetubular member and a bolt which can either be a standard bolt with areusable flare washer or a specially manufactured bolt having anintegral flare portion. Reinforcing is accomplished without the need forwelding, heat treating, soldering or any other action. Only two holedrilling operations are required, that is, the drilling of an in-linehole followed by the drilling of a coaxial enlargement hole followed bythe insertion of a sleeve, a bolt and a nut which is tightened tocomplete the reinforcing operation. Several methods may be used toensure that the nut does not back off during subsequent operations.

The claims and the specification describe the invention presented andthe terms that are employed in the claims draw their meaning from theuse of such terms in the specification. The same terms employed in theprior art may be broader in meaning than specifically employed herein.Whenever there is a question between the broader definition of suchterms used in the prior art and the more specific use of the termsherein, the more specific meaning is meant.

While the invention has been described with a certain degree ofparticularity, it is manifest that many changes may be made in thedetails of construction and the arrangement of components withoutdeparting from the spirit and scope of this disclosure. It is understoodthat the invention is not limited to the embodiments set forth hereinfor purposes of exemplification, but is to be limited only by the scopeof the attached claim or claims, including the full range of equivalencyto which each element thereof is entitled.

What is claimed is:
 1. A method of reinforcing a sandwich panel, thepanel having a first exterior skin, an intermediate skin and a secondexterior skin, the skins being separated by first and second cores, theexterior skins each having an external surface and an inner surface,comprising:(1) boring an in-line hole through the panel to penetrate thefirst exterior, the intermediate and the second exterior skins and thepanel cores providing a hole in the cores and the three skins ofdiameter A; (2) enlarging the hole in said first and second exteriorskins to a diameter B; (3) inserting a first tubular sleeve into saidhole in said first exterior skin, the first sleeve having a first endand a second end and a length substantially equal to the spacing betweensaid first exterior skin and said intermediate skin; (4) inserting asecond tubular sleeve into said second exterior skin, the second sleevehaving a first end and a second end and a length substantially equal tothe spacing between said second exterior skin and said intermediateskin, the second sleeve being in axial alignment with said first sleeve,the first sleeve each having a diameter greater than A and slightly lessthan B, the first end of each of the sleeves contacting a surface ofsaid intermediate skin; (5) flaring said second end of each of saidfirst and second sleeves to an external diameter greater than B, saidsecond end of each of said sleeves contacting respectively said innersurfaces of said first and second exterior skins.
 2. The methodaccording to claim 1 including:(5) inserting a bolt through said tubularsleeves, the bolt having a head of diameter greater than B, and the headcontacting said exterior surface of said first exterior skin; and (6)installing a nut on said bolt to contact said exterior surface of saidsecond exterior skin.
 3. The method according to claim 2 wherein thestep of flaring said second end of each of said first and second sleevesincludes positioning on said bolt first and second flare washers, oneadjacent said bolt head engaging said first tubular sleeve second endand one adjacent said nut engaging said second tubular sleeve secondend.
 4. The method according to claim 1 wherein step (5) comprisesinserting a bolt into said aligned tubular sleeves, the bolt having anenlarged diameter head and a frusto-conical portion adjacent the head,followed by installing a nut on said bolt, and tightening the nut onsaid bolt to cause said frusto-conical portion to flare said second endof said first sleeve.
 5. The method according to claim 4 wherein saidfrusto-conical portion is removably positionable on said bolt.
 6. Themethod according to claim 5 wherein said frusto-conical portionremovablypositionable on said bolt includes an integral washer portionthat contacts said external surface of said first skin 12.